The present invention relates to computer printers and particularly to a printer carriage control apparatus for horizontally positioning the printer carriage and print head during a printing operation. In computer printers, characters are formed on paper by the print head which is mounted on the carriage. The carriage travels horizontally back and forth across the face of the paper. In order to provide the proper horizontal spacing between and/or within characters during printing, it is necessary to position accurately the printer carriage along its horizontal path of travel.
Accurate horizontal positioning of the carriage and print head is important to provide an aesthetically pleasing printed page. Moreover, accurate positioning is especially important where the printer is used to fill in the "blanks" on pre-printed forms. Inaccurate horizontal positioning of the print head can cause the print characters to miss the "blank" space on pre-printed forms thereby producing overstriking of the pre-printed material and a generally unacceptable printing face.
Computer printers, that have been known and used in the past, have relied on several schemes for providing horizontal positioning of the printer carriage and print head. One scheme is the so-called "picket fence" arrangement. An encoding strip of alternate, evenly spaced dark and light bars is affixed to the frame of the printer and extends parallel to the horizontal path of travel of the printer carriage. The alternate dark and light bars of the encoder strip have the appearance of a picket fence and thus the terminology to describe such an arrangement. The printer carriage, in the picket fence arrangement, carries with it a light source and photosensor which is alternately activated and deactivated by light reflected from or transmitted through the light and dark horizontal bars of the "picket fence" as the printer carriage travels back and forth across the face of the paper. By counting the number of bars, the picket fence arrangement can establish through its logic control circuitry its position along its horizontal path of travel and, therefore, provide the requisite horizontal positioning of the printer carriage.
The picket fence provides an accurate system for horizontal positioning of the print head, but it is susceptible to drawbacks. Because the picket fence extends across the entire width of the computer frame, it is necessarily in the open, near where the paper is being fed into the printer and where impact printing is actually being accomplished. The loose paper fibers and other dirt that result from the print head impact and the feeding of paper at high speed can settle onto the picket fence and mask its alternate dark and light alternating bars. As a result, the photosensor in the carriage is not able to provide an accurate count of the bars in the picket fence and thus cannot provide an accurate position for the carriage. Moreover, at best, the positioning of the carriage is in a one-to-one ratio in that the resolution of the picket fence is dependent on a photosensor's ability to resolve the smallest dimension of the alternative bars of the picket fence.
Another known arrangement for positioning a printer carriage and print head is by providing an encoder disk attached (either directly or through gearing) to the carriage drive motor. The encoder disk has evenly spaced rectangular openings positioned around its periphery. A light source is placed on one side of the encoder disk and a photosensor is placed on the other side of the encoder disk. The photosensor detects the openings in the disk, and the printer's logic circuit converts the photosensor signal into a count. The resultant count is proportional to the rotational displacement of the encoder disk, which is in turn proportional to the rotational displacement of the carriage drive motor. The carriage is connected to the drive motor by means of the continuous, looped cable or belt transmission system so that the carriage's horizontal position is theoretically proportional to the rotational displacement of the drive motor and the encoder disk. Thus by monitoring the encoder disk's rotation, the printer's control logic knows where the carriage is located.
The encoder disk attached to the motor, can provide for a resolution ratio greater than one-to-one by means of providing a gearing between the motor drive and the encoder disk. In that regard, the encoder disk mounted on the carriage drive motor has an advantage over the picket fence arrangement.
The principal drawback to the encoder disk mounted on the motor drive is that between the encoder disk mounted near the drive motor and the carriage itself is a mechanical drive train consisting of the continuous looped cable or belt extending across the full width of the printer frame. As a result any stretching or backlash in that drive cable or any eccentricities in the pulleys on which the drive cable is mounted or any varying diameters in the drive pulleys all represent errors which can affect the accurate positioning of the carriage and print head.
For example, the backlash in the cable is particularly troublesome because the stretching and contraction of the cable relates to the inertia and speed of the movement of the carriage itself. As a result, during a continuous printing operation across a page, the carriage may be accurately positioned horizontally, whereas when the carriage is moved across a page in a long travel followed by a short burst of printing followed by another long travel, the backlash can significantly change the positioning of the print head for a given encoder disk rotational displacement.
Another conventional carriage control scheme is a so-called time-controlled system which utilizes a timer in conjunction with a stepping motor to establish the print head's horizontal position.